To aid with the migration of these classes to Kotlin (see
servo#45640), restricting the visibility
of these symbols means that the Kotlin translation can be more faithful
without looking odd (e.g., converting modifier-less fields to `internal`
when they don't need to be `internal` would look odd in Kotlin).
Testing: There are no automated tests for Android.
Signed-off-by: veyndan <veyndan@gmail.com>